Recently, in optical communications, research and development of the optical wavelength division multiplexing communication have been conducted actively for the way to dramatically increase the transmission capacity thereof, and practical applications have been proceeding. The optical wavelength division multiplexing (WDM) communication is that a plurality of lights having a wavelength different from each other is wavelength-multiplexed and transmitted, for example. In such optical wavelength division multiplexing communication systems, lights need to be drawn out of the multiplexed light to be transmitted at every wavelength on the light receiving side. On this account, light transmitting elements that transmit light having only a predetermined wavelength need to be disposed in the systems.
As one example of the light transmitting elements, for example, there is an arrayed waveguide grating (AWG) as shown in FIG. 6. The arrayed waveguide grating is an optical waveguide circuit chip where a waveguide forming region 10 is formed on a substrate 1 such as silicon. The waveguide forming region of the arrayed waveguide grating has the following waveguide configuration.
The waveguide configuration of the arrayed waveguide grating is formed to have one or more of optical input waveguides 2; a first slab waveguide 3 connected to the output ends of the optical input waveguides 2; an arrayed waveguide 4 made of channel waveguides 4a arranged side by side, the channel waveguides being connected to the output end of the first slab waveguide 3; a second slab waveguide 5 connected to the output end of the arrayed waveguide 4; and a plurality of optical output waveguides 6 arranged side by side, the optical output waveguides being connected to the output end of the second slab waveguide 5. Additionally, the optical input waveguides 2 are also arranged side by side in plurals in FIG. 6.
The channel waveguides 4a are for propagating light that have been lead through the first slab waveguide 3, which are formed to have a length different from each other by a set amount. The length of adjacent channel waveguides 4a varies from each other by xcex94L. The channel waveguides 4a that constitute the arrayed waveguide 4 are generally disposed in multiple such as a hundred. However, in the same drawing, the number of the channel waveguides 4a is schematically depicted to simplify the drawing.
Furthermore, the optical output waveguides 6 are disposed corresponding to the number of signal lights having a different wavelength each other, the signal lights are demultiplexed or multiplaexed by the arrayed waveguide grating, for example. However, in the same drawing, the number of each of the optical output waveguides 6 or the optical input waveguides 2 is schematically depicted to simplify the drawing.
To the optical input waveguides 2, for example, optical fibers on the transmitting side (not shown) are connected to lead wavelength-multiplexed light. The light that has been lead to the first slab waveguide 3 through the optical input waveguides 2 spread by the diffraction effects thereof to enter each of the channel waveguides 4a, propagating through the arrayed waveguide 4.
The light that has propagated through the arrayed waveguide 4 reach the second slab waveguide 5. Then, they are condensed at the optical output waveguides 6 to be outputted. The length of the entire channel waveguides 4a that constitute the arrayed waveguide 4 varies from each other by the set amount. Thus, a shift is generated in the phase of the respective lights after propagating through the arrayed waveguide 4. According to this shifted amount, the phasefront of the light is tilted. The positions at which the light is condensed are determined by this tilted angle.
Therefore, the positions at which the lights having a different wavelength are condensed differ from each other. The optical output waveguides 6 are formed on the positions and thereby the lights having a different wavelength can be outputted from the different optical output waveguides 6 at every wavelength.
Additionally, the arrayed waveguide grating utilizes the principle of reciprocity (reversibility) of optical circuits. Thus, it has the function of an optical multiplexer as well as the function of an optical demultiplexer. That is, when a plurality of lights having a different wavelength enters from each of the optical output waveguides 6, these lights pass through a propagation path reverse to that described above, are multiplexed by the second slab waveguide 5, the arrayed waveguide 4 and the first slab waveguide 3 and are emitted from one of the optical input waveguides 2.
In such the arrayed waveguide grating, the wavelength resolution is in proportion to a length difference (xcex94L) of the adjacent channel waveguides 4a constituting the arrayed waveguide 4, as described above. Therefore, in the arrayed waveguide grating, the xcex94L is designed large and thereby the optical multiplexing/demultiplexing of light(s) having a narrow wavelength spacing is made possible, which could not be realized by an conventional optical multiplexer/demultiplexer. For example, the xcex94L is formed large to design a designed wavelength spacing to be 1 nm or less and thereby the function of demultiplexing or multiplexing a plurality of light signals having a wavelength spacing of 1 nm or less can be served. That is, the arrayed waveguide grating can serve the function of multiplexing and demultiplexing a plurality of signal light (s), which is needed to realize the high-density optical wavelength division multiplexing communications.
The waveguide forming region 10 of the arrayed waveguide grating described above has an under cladding layer, a core layer and an over cladding layer. The under cladding layer, the core layer and the over cladding layer are made of silica-based glass. The arrayed waveguide grating is formed by forming the under cladding layer on the substrate 1, forming the core layer having the waveguide configuration described above thereon and forming the over cladding layer that covers the core. Additionally, in the conventional arrayed waveguide grating, the over cladding has been formed of silica-based glass of pure silica glass added with 5 mol % of each of B2O3 and P2O5.
And now, in the arrayed waveguide grating as described above, it is known that polarization dependent loss is generated because an effective index of the TE mode differs from an effective index of the TM mode in the lights propagating through the arrayed waveguide 4.
Then, in order to compensate polarization dependent loss, a half waveplate 18 is traditionally disposed so as to cross the longitudinal center part of the entire channel waveguides 4a as shown in FIG. 6. Additionally, in the same drawing, a slit 17 is formed in the longitudinal center part of the arrayed waveguide 4 so as to be orthogonal to the arrayed waveguide 4. The half waveplate 18 is inserted into this slit 17 and is disposed orthogonal to the arrayed waveguide 4.
The half waveplate 18 disposed in the longitudinal center part of the arrayed waveguide 4 converts the TE mode to the TM mode or inversely converts the TM mode to the TE mode before and after the half waveplate 18. According to this conversion, the difference of the optical path between the TE mode and the TM mode (physical lengthxc3x97effective index) that has been generated before the light propagated through the arrayed waveguide 4 enter the half waveplate 18 is cancelled before propagating to the output end of the arrayed waveguide 4 and thus the polarization dependent loss is compensated.
An optical waveguide circuit module of the invention comprises:
an optical waveguide circuit chip comprising a waveguide forming region made of silica-based glass formed on a substrate; and
a housing for accommodating the optical waveguide circuit chip,
wherein a water-insoluble oil is filled inside the housing.